This invention relates to the heating of materials within a cavity by use of electromagnetic energy and, more particularly, to an electromagnetic feed system for vulcanizing rubber tires in a mold.
Rubber tires are employed on a variety of vehicles ranging in size from the relatively small automobiles to the relatively large earth moving equipments. While the thickness of the side walls of rubber tires is sufficiently thin to admit a uniform heating of the side walls by the conduction of heat thereto from the heated walls of a mold, the tread region of such tires, particularly the larger tires of earth moving equipments, is very much thicker with the result that the thermal insulating properties of the rubber greatly reduce the amount of heat conducted into the interior portions of the tread as compared to the amount of heat conducted into the exterior portions of the tread contiguous to a heated wall of the cavity.
One approach to the acceleration of the vulcanizing process is the use of microwave energy for heating the rubber. As is well known, microwave energy has been utilized for heating a large number of substances, and has found extensive use in microwave ovens for heating food. A problem arises in the use of microwave energy in an enclosed metallic cavity in that standing waves of reflected energy introduce local variations in the intensity of the energy with the result that non-uniform heating of the material in the cavity results. While devices such as mode stirrers comprising moving reflecting elements may be employed in microwave ovens, such devices are impractical within the metallic mold utilized in forming a tire because of the intricate shape of the mold as well as the fact that a substantially large portion of the cavity is filled with the material, namely the rubber, which is to be heated.
One attempt at the solution of the foregoing problem by the utilization of microwave energy for heating material contained in a metallic cavity, such as the heating of a rubber tire, is disclosed in the U.S. Pat. No. 2,738,406 which issued in the name of Zaleski on Mar. 13, 1956. Therein, microwave energy is introduced via a tortuous path following a metallic surface of an air bladder utilized in inflating the tire against the inner surface of the mold. A problem arises therein in that it is difficult to maintain a uniform intensity of electromagnetic power since the power is progressively attenuated by the rubber as the radiant energy propagates along the tortuous path.
An additional problem becomes evident when an attempt is made to direct electromagnetic energy into the cavity by a radiation thereof. The wavelength of electromagnetic energy utilized in heating operations is generally greater than or equal to approximately 15 centimeters. Accordingly, a waveguide of suitable dimensions to propagate such radiant energy would be difficult to place within the smaller tire molds due to a lack of space. And even in the case of a larger tire molds, if it be desired to produce a radiator having a radiating aperture of many wavelengths to produce a highly directive beam of radiant energy, there would be a lack of space so that typical radiation patterns associated with the far field of a radiator could not be accommodated.